When used to charge an imaging member, a roller used to create a charge on another surface or substrate is commonly referred to as bias charge roll (“BCR”). When used to charge a substrate to enable transfer of a developed image from an imaging member to a substrate member, a roller used to create such bias charging is commonly referred to as a bias transfer roll (“BTR”). Although both may differ in details particular to their applications, both represent illustrative embodiments of the present invention.
Generally, the process of electrostatographic reproduction is initiated by substantially uniformly charging a photoreceptive member, followed by exposing a light image of an original document thereon. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface layer in areas corresponding to non-image areas in the original document, while maintaining the charge on image areas for creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which a charged developing material is deposited onto the photoconductive surface layer, such that the developing material is attracted to the charged image areas on the photoreceptive member. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or some other image support substrate to which the image may be permanently affixed for producing a reproduction of the original document. In a final step in the process, the photoconductive surface layer of the photoreceptive member is cleaned to remove any residual developing material therefrom, in preparation for successive imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for both digital copying and printing as well as for light lens copying from an original. In many of these applications, the process described above operates to form a latent image on an imaging member by discharge of the charge in locations in which light from a lens, laser, or LED discharges a charge. Such printing processes typically develop toner on the discharged area, known as DAD, or “write black” systems. Light lens generated image systems typically develop toner on the charged areas, known as CAD, or “write white” systems. The embodiments of the present invention apply to both DAD and CAD systems.
With respect to BCR applications, those skilled in the art recognize that various devices and apparatus have been proposed for creating a uniform electrostatic charge or charge potential on a photoconductive surface prior to the formation of the latent image thereon. Generally, corona generating devices are utilized to apply a charge to the photoreceptive member. In a typical device, a suspended electrode, or so-called coronode, comprising a thin conductive wire is partially surrounded by a conductive shield with the device being situated in close proximity to the photoconductive surface. The coronode is electrically biased to a high voltage potential, causing ionization of surrounding air which results in the deposit of an electrical charge on an adjacent surface, namely the photoconductive surface of the photoreceptive member. Corona generating devices are well known, as described, for example, in U.S. Pat. No. 2,836,725, to R. G. Vyverberg, among numerous other patents and publications. In the referenced Vyverberg patent, the coronode is provided with a DC voltage, while the conductive shield is usually electrically grounded and the photoconductive surface to be charged is mounted on a grounded substrate, spaced from the coronode opposite the shield. Alternatively, the corona device may be biased in a manner taught in U.S. Pat. No. 2,879,395, wherein the flow of ions from the electrode to the photoconductive surface is regulated by an AC corona generating potential applied to the conductive wire electrode and a DC potential applied to the conductive shield partially surrounding the electrode. The DC potential allows the charge rate to be adjusted, making this biasing system ideal for self-regulating systems. Various other corona generating biasing arrangements are known in the art and will not be discussed in great detail herein.
Several problems have historically been associated with corona generating devices. One problem includes the use of very high voltages (3000-8000 V), requiring the use of special insulation, inordinate maintenance of corotron wires, low charging efficiency, the need for erase lamps and lamp shields and the like, arcing caused by non-uniformities between the coronode and the surface being charged, vibration and sagging of corona generating wires, contamination of corona wires, and, in general, inconsistent charging performance due to the effects of humidity and airborne chemical contaminants on the corona generating device. More importantly, corotron devices generate ozone, resulting in well-documented health and environmental hazards. Corona charging devices also generate oxides of nitrogen which eventually desorb from the corotron and oxidize various machine components, including the photoreceptor, resulting in an adverse effect on the quality of the final output print produced thereby.
As an alternative to corona generating devices used in charging systems, roll charging systems such as, BCR's and BTR's have been developed and incorporated into various machine environments with limited success. BCR charging systems are exemplified by U.S. Pat. No. 2,912,586, to R. W. Gundlach; U.S. Pat. No. 3,043,684, to E. F. Mayer; U.S. Pat. No. 3,398,336, to R. W. Martel et al.; U.S. Pat. No. 3,684,364, to F. W. Schmidlin; and U.S. Pat. No. 3,702,482, to Dolcimascolo et al., among others, wherein an electrically biased charging roller is placed in contact with the surface to be charged, e.g. the photoreceptive member. Also relevant is U.S. Pat. No. 5,412,455, to Ono et al. wherein a charging device includes: a member to be charged; a charging member connectable to the member to be charged; a power source for supplying an oscillating voltage to the charging member; and a constant voltage element connected electrically in parallel with the power source for generating the oscillating voltage. Also, U.S. Pat. No. 5,463,450, to Inoue et al. discloses a charging apparatus for electrically charging a member to be charged including a charging member contactable to the member to be charged. The member to be charged includes a core and a voltage source for applying an oscillating voltage between the member to be charged and the charging member, wherein the frequency of the oscillating voltage satisfies a predetermined condition. Each of these is hereby incorporated by reference in their entirety.
In BTR charging systems, DC voltage is typically used. DC voltage attracts dirt, however, especially toner in spaces void of printing substrates, such spaces comprising inter-document zones, areas exposed when printing on less-than-full-width printing media, and similar areas in which the BTR is directly exposed to the charge carrying member or intermediate transfer member. Paper debris is also another contaminant of BTR systems. In response, conventional BTR apparatus require brushes to remove dirt and debris. Such brushes, however, add cost and complexity, occupy valuable space, and require maintenance when clogged or filled with dirt.
In BCR charging systems, a charging member in the form of a roller is contacted with the surface of the photoreceptive member or other member to be charged, and an oscillating input voltage, typically a DC biased AC voltage signal, is applied to the roller to generate an oscillating electric field for applying a charge potential of a given polarity, to the photoreceptive member where the DC offset defines the polarity of the charge applied. Although the input voltage may be comprised solely of a DC component, an oscillating voltage such as, an AC voltage signal having a DC voltage signal superimposed thereon has been found to be preferable with respect to charge uniformity. See, e.g., U.S. Pat. No. 4,851,960 to Nakamura et al which teaches that peak-to-peak input voltage, Vp-p, for DC-biased AC wave form should be twice the charge starting voltage for the photoreceptor or other charge receptor in the system being charged.
The absence of charge uniformity tends to manifest itself in the form of periodic stripes or so-called strobing corresponding to the variation in charge potential on the photoconductive surface. This strobing effect causes variations in toner attraction during development and often results in significant image quality degradation. However, an oscillating input voltage contributes both positive and negative polarity charge to the photoconductive surface. This results in a charging system that requires relatively high charging currents which, in turn, has a negative effect on the functional life of the photoreceptive member. Thus, a significant disadvantage of most biased roll charging systems is the resulting rapid wear of the photoconductive surface caused by the electrical discharge from the bias charge roll during the charging process. A related cause for rapid wear appears to be chemical degradation of organic and other complex molecules coupled with repetitive wiping or scraping of the photoreceptor layers by cleaning blades or other cleaning members.
One partial solution to the above problems is found in U.S. Pat. No. 5,613,173, issued to Kunzmann et al., hereby incorporated by reference in its entirety. In Kunzmann, a BCR apparatus is disclosed having clipped AC input voltage to reduce the phenomenon of strobing while also reducing photoreceptor wear caused by the electrical discharge from the bias charge roll during the charging process. The clipping of the AC oscillating voltage removes one polarity from the input signal, thereby supplying a single polarity to the photoreceptor or other charged member and, as a result, enabling sufficient charging at lower voltages applied to the charged surface. Such lower voltages extend photoreceptor life, in part by reducing electrically induced chemical damage.
Although the solution of Kunzmann improves photoreceptor wear, elimination of an AC oscillating voltage leaves only DC voltage, and DC-only voltage attracts dirt such as, toner particles, dust, and paper debris. One solution is to add cleaning apparatus such as, brushes and cleaning blades. These, however, add cost, complexity, and maintenance issues. What is needed is a system that retains the improved durability advantages of a single polarity waveform as well as the cleaning advantages of a multiple polarity waveforms.
In accordance with one embodiment of the present invention, an apparatus for applying an electrical charge to a member to be charged is provided, said charging apparatus comprising: a power supply for supplying an oscillating voltage signal; a charge roll member situated in proximity to a surface of the member to be charged; and a switch for switching between a plurality of modes wherein: (a) in a first mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including a single polarity input drive voltage to said charge roll member; and (b) in a second mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including an oscillating voltage signal containing multiple polarity components in order to supply oscillating polarity input drive voltage to the charge roll member.
In accordance with another embodiment of the invention, an electrophotographic imaging system is provided, said imaging system comprising: an apparatus for applying an electrical charge to a member to be charged, said charging apparatus comprising: a power supply for supplying an oscillating voltage signal; a charge roll member situated in proximity to a surface of the member to be charged; and a switch for switching between a plurality of modes wherein: (a) in a first mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including a single polarity input drive voltage to said charge roll member; and (b) in a second mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including an oscillating voltage signal containing multiple polarity components in order to supply oscillating polarity input drive voltage to the charge roll member.
In accordance with another embodiment of the invention, a process for applying an electrical charge to a member to be charged is presented, said process comprising: supplying an oscillating voltage signal from a power supply; positioning a charge roll member proximately to a surface of the member to be charged; and selecting between a plurality of modes, wherein: in the first mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including a single polarity input drive voltage to said charge roll member; and in a second mode, an electrical bias is applied from the power supply to the charge roll member, the electrical bias including an oscillating voltage signal containing multiple polarity components in order to supply oscillating polarity input drive voltage to the charge roll member.